As one of the standards for the mobile communication system, there is a standard called CDMA2000. To CDMA2000 is applied HDR (High Data Rate) which uses a dedicated frequency band for data communication in addition to a dedicated frequency band for voice communication. In HDR, a shared channel in which the dedicated frequency band (channel) for data communication is shared by a plurality of users is used.
The shared channel is defined as a channel in which data transmission rate (hereinafter, simply referred to as “rate”) is appropriately changeable, and the same frequency is used by a plurality of mobile station apparatuses wherein only one mobile station apparatus that has been selected by a base station apparatus is permitted to use exclusively the frequency in time-sharing mode to carry out data communication.
In a conventional mobile communication system, which uses such shared channel as described above, first, a plurality of mobile station apparatuses receive a pilot burst signal transmitted from a base station apparatus. Then, each mobile station apparatus measures CIR (Carrier to Interference Ratio), which represents the reception quality, based on the received pilot burst signal, and transmits a rate request value according to the CIR measurement value to the base station apparatus. At this time, transmission of the rate request value is carried out from every mobile station apparatus within the communication area at the same timing. Then, the base station apparatus, on receiving the rate request value from each mobile station apparatus, selects a mobile station apparatus to which data is to be transmitted based on the received rate request values, and transmits the data to the relevant mobile station apparatus at a rate of the rate request value received from the selected mobile station apparatus.
However, in the conventional system, since the data is transmitted only to the mobile station apparatus that has been selected by the base station apparatus, the mobile station apparatus, even when it transmits the rate request value, can not receive the data unless it is selected by the base station apparatus, and thus needs to transmit the rate request value continuously until it is selected by the base station apparatus.
That is to say, when the rate request value transmitted from the mobile station apparatus is smaller than a rate assigned to the data transmission by the base station apparatus, the relevant mobile station apparatus is rarely selected but nevertheless needs to transmit the rate request value continuously, thus resulting in a problem of useless electric power consumption for that continuous period of time. Since the power consumption for the transmission in the mobile station apparatus is large, a battery runs out earlier due to useless power consumption.
Further, when the mobile station apparatus is in a standstill or moving at a low speed, the mobile station apparatus transmits every time the same rate request value, even though CIR fluctuates little. Therefore, in this case also, there is such a problem of useless electric power consumption.